George.Yes, Gladys.There's a man at the door with a moustache.Tell him I've already got one. . . All right, all
right. What's he want then?He says do we want a documentary on mollusks.Mollusks!Yes.What's he mean, mollusks?
[screaming] MOLLUSKS!! GASTROPODS!
LAMELLIBRANCHS! CEPHALOPODS!Oh mollusks. I thought you said bacon. . . All
right, all right. What's he charge then?It's free.

These pages constitute an attempt
to fill a void in popular materials covering
worldwide molluscan shells. An abundance of
literature, from small field guides to gorgeously
produced coffee-table books, is available for marine
shells. For land snails there is Abbott's 1982 Compendium
of Landshells. With few exceptions these
resources have excluded freshwater aquatic
mollusks. Much of the existing literature is
technical, covering a limited taxonomic or geographic
range. The material presented here is organized
in a manner typical of Abbott -- zoologically down to
the family level, then (where I have sufficient
material) regionally. I would like to
acknowledge the assistance of Paul Parmalee of the
University of Tennessee, and Alan Gettleman for
assistance in identification, Karl-Otto Nagel of the
Senckenberg Naturmuseum, Frankfurt for specimens and
help with European species and systematics, Stephanie
Clark, Marc Keppens, and others for providing
specimens from worldwide locations, Thomas Eichhorst
for images and information on the family Neritidae,
and Lynn Scheu and Mike Emery for helpful comments
and suggestions.

Classification

Freshwater mollusks, being
thoroughly polyphyletic, cannot be classified
independently of the rest of the great invertebrate
phylum mollusca. Below is an evolutionary tree
diagram for the mollusca modified from Brusca and
Brusca (1990), p 762. Arrangements for the
gastropods and pelecypods, and lower taxonomic
rankings with significant representation in fresh
waters, are reflected in the linked pages.

In
addition to standard taxonomic
classification, other schemes have been
devised that consider ecological or
biogeographical factors, the degree of
evolution that a taxon has undergone since
its ancestral invasion of fresh water, and
how salt-tolerant an otherwise freshwater
group may be. The following zoogeographic
classification is abstracted from
Banarescu (1990). He adapts an earlier
system used for freshwater fishes, and
generally based on salt tolerance, to
mollusks. The groupings include:

Primary
(dispersal confined to freshwater
continental routes)

Secondary
(freshwater fish with a range that
would require some exposure to
seawater)

"Primary" and
"secondary" are merged into
"continental" where
applied to freshwater prosobranch snails.
Freshwater pulmonate snails, which did
not migrate from salt water, are not included
in the following groupings.

Peripheral
gastropods (snails). Neritina
and other freshwater members of Neritidae,
members of the primarily marine Stenothyridae,
Iravadiidae, Tornidae, Assimineidae,
Hydrococcidae, Littorinidae, Truncatellidae;
and some of the Hydrobiidae. Melanopsidae
(Asian and New Zealand snails). Thiaridae
(parthenogenic "hornshells",
tropical) are also included. There are
freshwater members of other marine families
including Buccinidae, Marginellidae,
Turbonillidae, Trochidae, Umbonidae,
Muricidae, Nassariidae, Epitoniidae,
and from the Systellommatophorans (an order
conataining certain tropical slugs, but not
most common garden slugs and snails), two
freshwater Onchidiidae.

Peripheral
bivalves. Freshwater members of the
families Arcidae, Mytilidae, Lyonsiidae,
Donacidae, Cardiidae, including
many geologically recent extinctions, and Mactridae.
Members of many other families may live in
brackish waters.

Other malacologists prefer
to categorize using the fossil record, or
degree of kinship with a marine
ancestor. One such system includes:

Paleolimnic
(higher taxa that have no marine
ancestors)

Mesolimnic
(higher taxa with distant marine
ancestors)

Neolimnic
(more recent marine ancestors)

Zoogeography

The geographical distribution of
freshwater mollusks has much to say specifically
about their evolutionary history and reflect more
general trends observed in zoogeography. First,
a common notion that these animals are present in the
creeks and rivers because "this area was once
covered by the sea" needs to be dispelled.
Continents have moved, and seas have gone, come, and
gone again since the ancestors of "primary"
freshwater mollusks first crawled up the rivers.
Freshwater molluscan evolution spans a good chunk of
geologic time, and their distribution bears witness
to a number of causes--some yet unknown.
Drainage divides represent substantial barriers to
these creatures. The more isolated two rivers are
from each other, or the more important the drainage
divide between them is, the fewer species and genera
their molluscan faunas have in common. Besides
saltwater seas, large trunk rivers also form
substantial barriers to the species of headwaters and
small tributaries. For example, although creeks in
the Missouri Ozarks share many things in common with
those in East Tennessee, and all are tributary to the
Mississippi River, there are few unionid mussels and
no pleurocerid snail species common to the two
regions.

Another general observation is the
lack of the strong arctic-to-tropics gradient of
increasing diversity, size, color, etc. This is in
contrast to many other living things, including
marine mollusks, where a huge diversity of large
showy species live in the tropics, while arctic
species are few, and often drab and chalky.
With the exception of three tropical snail
families Neritidae, Thiaridae and Ampullariidae, most
freshwater families achieve their greatest size and
diversity in temperate, or even cold parts of the
world. There are no freshwater equivalents to
the conchs, cowries, and giant clams of marine
tropical waters.

Several authors have observed
similarities between the unionacean pearly mussels of
Australia and South America, noting that they have
more in common with each than they have with species
from the northern hemisphere. Prior to the
development of plate tectonic theory in the 1960s,
various explanations were proposed, including land
bridges, foundered beneath the waves long ago, or
ancestral migrations to both areas from the north,
with the fauna subsequently dying out in their native
(norther hemnisphere) lands. Had researchers
written just a few years later, this would have been
seen as another confirmation of plate tectonics, with
southern freshwater mussels survivors of the breakup
of Gondwana.

In eastern North America, the
Pleuroceridae (among the gastropods) and the
Unionidae tend to have endemic species with
restricted ranges. Species composition will change
from one river or creek to another. Where ranges
overlap, as in the case with three Elimia
snails surveyed in one area, they seem to be present
semi-continuously, with a single widespread species,
and one of the other two. Only rarely were all three
species present (personal observation). The
Hydrobiidae seem to be discontinuous, with
concentrations of a single species locally, and wide
areas where none are present.

Pilsbry and Becquaert (1927),
describing the zoogeography of African freshwater
mollusks, observe that many species and genera are
widespread in the Congo Basin and its vicinity. A
major exception to this is Lake Tanganyika, with its
abundance of strange endemic snails, some of which
closely resemble marine shells. They classify this
lacustrine fauna into three groups. Their group 1
included nonendemic species, and species belonging to
genera found in surrounding waters. Most such
mollusks live in shallow lagoons near the lakeshore.
Group 2 included strictly endemic genera, though they
are still obviously related to the surrounding fauna.
These generally live in the shallower waters of the
lake. Finally, group 3 includes the
"thalasoid" [Greek, Thalassa, sea]
snails, a unique and extremely varied assemblage of
forms. These are so different from freshwater snails
elsewhere in Africa that "their relationship can
only be traced through a careful study of their soft
parts and anatomy, and is in some cases still open to
question." (Pilsbry and Becquaert, 1927). Group
3 species are adapted to diverse habitats including
the wave-washed and algae covered rocks of the shore,
the shallower waters of the lake where they coexist
with snails and mussels of group 2, but also deeper
waters to 200 meters down, below which there is
insufficient oxygen. Pilsbry and Becquaert suggest
that the mollusk fauna of Lake Tanganyika resulted
from a few generalized freshwater types finding their
way into the lake and evolving in isolation over
time.

Geomorphologic aspects such as
stream capture or piracy may explain the zoogeography
for some groups. For instance, many pearly
Elliptio mussels live in Atlantic Coastal Plain
rivers of the southeastern United States-- some
endemic, others wide-ranging-- while only two species
exist in the continental interior, west of the Great
Eastern Divide. Of possible explanations, two
are likely. One involves an origin in the
American interior (the heartland of unionid
diversity generally), with escape and subsequent
adaptive radiation in the many parallel but separate
river systems of the Atlantic Coastal Plain.
The second involves a long period of speciation along
the Atlantic coast (the center of Elliptio's
diversity), with the eventual escape of two species
back across the Divide to the continental interior.

Importance to man

The importance of freshwater
mollusks to humans and the environment is greater
than their general absence in collections and popular
guides would lead one to believe, although with
increasing environmental awareness, and threats of
extinction, this lately has started to change.
Some negative aspects, where they are a source of
problems themselves, will be considered first:

Parasite vectors:While
landsnails and slugs may play minor roles in the
transmission of certain tapeworms and nematodes, the
importance of freshwater mollusks as intermediate
hosts for a great variety of human and animal
parasitic flatworms cannot be ignored.
Prosobranch gastropod taxa known to be involved in
parasite life cycles include Pila, snails
belonging to the Hydrobiidae and related families,
Thiaridae including Thiara, Pleuroceridae
including Elimia (Juga), and Semisulcospira,
and also several marine gastropods in the related
family Potamididae. Pulmonate gastropod hosts
include the Planorbidae, Bulinidae (now included
within the Planorbidae), and the Lymnaeidae (Chandler
and Clark, 1961).

Invasive species:The invasion of North
America by the zebra mussel has had catastrophic
consequences in some places. Their colonization
of Unionid mussel habitat and the shells of living
native mussels has been lethal to the native
inhabitants. In the Illinois River, a mass
die-off of zebra mussels in the summer of 1994
resulted in the death of large numbers of fish and
native Unionid mussels. Populations of zebra
mussels had exploded to densities of over 60,000 per
square meter, a concentration of biomass that could
not be sustained by available oxygen during summer
low-water. Mortalities then precipitated a
snowballing effect.

Previous displacement of native
American species by the Asiatic clam Corbicula
proceeded more slowly. Both Corbicula and more
so, the zebra mussel, have the capability of fouling
up water intake systems for power plants and
municipal purification plants.

Passive indicators of
environmental degradation:The news they tell us is
bad. Besides suffering from natural calamities
(both drought, and floods when augmented by
anthropogenic erosion and siltation), a number of
species in North America have become endangered, or
been driven to extinction due to the fourfold attack
of pollution, siltation due to agricultural runoff,
river impoundment, and invasive exotic species.
The Unionidae were among the first victims of
widespread pollution. For example, in 1922 it
was observed that the mussel fauna in the Big
Vermillion River had been completely destroyed for a
distance of 14 miles below the sewage outfall of
Urbana, Illinois (Parmalee, 1967).

Siltation from agricultural runoff
and other disturbances harms freshwater species
directly through smothering them, covering them with
silt, and altering the habitat by covering what was
formerly clean sand, gravel, and algae-covered stones
with anywhere from a thin layer to many meters of
silt.

Dam construction changes habitat
radically, both above and below the dam.
Populations in the free-flowing reaches above the
impoundment become isolated. In the reservoir
itself, deep quiet waters no longer favor the
abundance and variety of species originally
present. Below the dam, released water may be
clear and clean, but with radically different
chemical and physical properties. This is
typical of deeper reservoirs, where cold winter
temperatures and low oxygen contents are retained in
the bottom waters and released through the turbines
year-round.

Shell: Freshwater shells
have always been present in the seashell trade,
though often not so labeled for the end user.
Their most important use economically was for the
manufacture of pearl buttons before the application of plastic in the
1940s and '50s. Presently, mussel shells are
exploited for use as pearl blanks in the Japanese
cultured pearl industry.

Foodstuff: Piles of mussel shells and the larger
freshwater snail shells are found along the banks and
in the soil near, and sometimes well away from, many
of the larger rivers throughout North America.
Extinct (recently, due to anthropogenic causes) or
regionally extirpated species are often
encountered. This suggests that aboriginal
cultures used freshwater mussels and snails as a
source of protein. In Africa, river oysters (Etheria)
were harvested and dried in the Belgian Congo, now
Zaire, later to be made into a stew. Neolithic
tribes living along the Nile had many uses for the
large Spathiopsis shells, and they form the
bulk of the Neolithic kitchen middens there.
They have also been recovered from many Egyptian
graves, their meaning and significance suggested by
the existence of a beautifully carved diorite
representation found in an Egyptian necropolis (Van
Damme, 1984).

Parmalee (1998) indicates that a
variety of methods may have been used in their
preparation in North America, but that Europeans
generally find them tough and completely
unpalatable. Shakespeare alludes to this in The
Tempest (Act 1, scene 2, lines 460-465
(Riverside edition)):

It should be noted that primitive
people often use elaborate methods to prepare and
process otherwise toxic or unpalatable
foodstuffs.

Gastropods have helped to feed many
cultures. Tribal elders in Zaire ate local Pila
snails ritualistically at certain times of the year,
and the leftover shells were worn as charms to
protect against drowning, or as headdresses assuring
fertility (Van Damme, 1984). In the Amazon basin,
large apple snails like Pomacea canaliculata
and P. bridgesii are raised, and their
mild-tasting muscular portions are eaten as the dish
"churo." Pila snails are
eaten raw by the Ilocano people of the Philippines,
but may be a source of parasitic infection (Chandler
and Clark, 1961). The Chinese Mysterysnail (Cipangopaludina)--collected
and offered for sale in San Francisco's and Boston's
traditional Chinese markets-- can also harbor
parasites.

In classical times, Pliny the Elder
indicates the flesh of "river snails" [fluviatilium
coclearum] was used raw or boiled, as an
antidote for scorpion venom. It could either be
eaten, or applied directly to the wound.

Finally, freshwater Corbicula
clams and various snails are used as foodby
Oriental peoples, and are now shipped overseas from
China and Viet Nam, and avialable in many Asian
grocery stores worldwide.

The Images

Most of the images are direct scans
of the shells. They were scanned at resolutions of
600, 300, 150,75 and 50
pixels per inch. This results in magnifications
close to 8, 4, 2,1, and 2/3
respectively when viewed on an 11 inch (28 cm) wide
computer monitor. [Click here for scales] In most cases this information is included
at the end of the filename, accessible by right
clicking the image, and going down the pop-up menu to
"properties." The "upside
down" perspective of the flat-bed scanner
introduces a distortion somewhat
different from other illustrations and
photographs. It is most apparent in
short-spired snail shells resting on their apertures,
and causes spires to appear foreshortened.